Hi Slim
I guess you take advantage of the many services provided by government and - indeed - by private enterprise.
It seems like you would be surprised to know that these censuses are absolutely vital for those services to plan for future demand, because future demand depends almost entirely on how many people there are, and what their general characteristics are - e.g. age/sex/location in particular. Some services - e.g. aged care - require more detailed data, such as how many people are aged and living with disabilities. Others - probably including you - would complain if the roads you need in future weren't constructed and maintained - but how on earth would those responsible for doing that know where these future roads should go, how much traffic they will carry etc?
As far as "not having a clue" - that is also way off the mark, as there are many ways of getting good estimates of groups such as "illegals". There are tried and true statistical techniques practiced by good statistical agencies all around the world. "Successive Labour and Tory PMs" rely on the Censuses to provide the very estimates you say they don't have. You're right - they have no idea -
until they do the Census.
Your criticism of the Census is - to say the least - uninformed! If you were complaining about how those in high places ignore the evidence collected by these surveys, then I would have a lot more sympathy for your opinions.
With respect!